There's a certain genius to the concept behind Liz Garbus' documentary about Marilyn Monroe, "Love, Marilyn," airing Monday on HBO. But the film itself is kind of a mess, albeit a mess you can't stop watching because, after all, it's Marilyn.

Anyone who dares a film about Monroe 50 years after her death is a brave soul indeed. What is left to be said? The answer in this case is a trove of Monroe's letters and diaries discovered only a few years ago. They are enlightening, fascinating, a performance of sorts and elusive, just like their author was in life and remains in posthumous mythology of singular endurance.

The brilliance of the concept, which is not unlike the approach that Todd Haynes took on the life of Bob Dylan in "I'm Not There," has to do with Monroe the performer. Pop a camera in her face, and she became a different entity. The late Susan Strasberg, daughter of Monroe's acting mentors, Lee and Paula Strasberg, is shown telling again the story of walking down the street with an unnoticed Monroe who asks if she'd like to see "her." In a flash, and without any change in costume, she allows the public's Marilyn Monroe to emerge, and she is instantly mobbed.

See the footage of Monroe besieged by the press after her divorce from Joe DiMaggio, or giving a press conference after her return to Hollywood from self-imposed exile in New York. Her lids float over her eyes, her mouth coos, the lips almost seem choreographed as she speaks so precisely in a voice that is breathy and almost overtly sexual.

Ironically, the few moments when an unguarded Monroe can be seen occur while she is about to film a scene in a movie, when no one is watching except the crew and the camera. She's natural, girlish, relaxed as she never seems to have been in her self-managed public role.

Just as Norma Jeane Mortenson enacted Marilyn Monroe, the actors in "Love, Marilyn," portray Monroe by reciting her diaries and letters. Some of the performances are fascinating and deft - Tomei's, for example, as well as Jennifer Ehle's and Close's. But others, such as Thurman's, just aren't very good. Monroe may have been acting whenever she was in public, but it was very good acting. It may be surprising to some, but Lohan's moments stand out, in part because of her own all-too-public persona. When she reads Monroe, we experience a strange and compelling awareness of both her public image and Monroe's iconography blending together.

The written material itself is fascinating, of course. There may not be any bombshell revelations here, but we get more of a sense of her trying to be authentic as she writes poetry, lists her goals in life and work, and records information about lesser-known Italian Renaissance painters she is studying. It's not coincidental that the writing becomes more worked-over the deeper she gets into psychoanalysis and dependence on prescription medication, reflecting her growing struggle to know what being authentic really meant.

Garbus was painted into a corner on this project from the beginning: You simply can't do anything on Marilyn Monroe, even readings of recently unearthed writing, without slamming into the mythology. But having all these actors trying to be Monroe is at times very confusing and even, at times, silly. At those moments, we're not reminded of the often fragile co-existence of the movie star and the yearning product of many foster homes in the same body but, rather, of the transparency of having other people speak her words.

Of course, the idea behind "Love, Marilyn" is to reinforce the obvious, that Marilyn Monroe cannot speak for herself. Fortunately, her written words are not entirely drowned out by the artificiality of the film's concept. The words are hers, and that counts for something.